An internal combustion engine (e.g., a diesel engine) typically comprises a crankcase having a breather and an oil pan. Crankcase ventilation fumes (e.g., fumes that leak past the piston rings and through circulating oil) leave the crankcase through its breather. A filter assembly, having a gas inlet connected to the crankcase breather, can filter the ventilation fumes to supply clean gas to engine's air intake line. This filter assembly can also separate oil from the filtered gas and return it the crankcase oil pan. And, if the filter assembly includes a pressure regulator, it can help counteract excessive negative crankcase pressures.